This invention relates to a method and device for measuring the environmental stress crack resistance of plastic articles.
An important aspect of the production and use of plastic articles is the evaluation of physical characteristics of articles made from plastics. Besides having applications in engineering and design, the data obtained from such evaluations may be used to monitor the quality of the plastic produced and the products thereof or to set specifications for acceptance testing of raw materials. It is known that certain polymers are sensitive to stress-cracking in the presence of certain chemical agents. For example, high impact polystyrene (HIPS), copolymers of acrylonitrile butadiene and styrene (ABS), and high impact polyvinyl chlorides are sensitive to stress-cracking in the presence of oils. Such an effect is known as "environmental stress cracking" or "environmental stress fracture." In the presence of stress cracking agents, plastic articles can undergo stress failure (cracking or crazing) at stress levels below those which would otherwise induce such failure. Evaluation of the ability of a particular plastic article to resist such environmental stress cracking is important since the end use of such a product may involve contact with certain materials known to promote stress cracking. Methods for evaluating environmental stress cracking have also been used in fundamental studies of polymer properties and behavior. It has been observed that certain polymer characteristics such as molecular weight and morphology affect the tendency of the plastic article to undergo environmental stress cracking.
An article by Titow in Plastics and Polymers (June 1975) discloses the known methods for determining the tendency of plastic articles to undergo environmental stress cracking. Tensile-load methods are known, including the so-called Lander test (ASTM D-2552) in which twenty waisted tensile specimens are immersed in a liquid medium under constant tensile load. Other tensile-load methods involve a specimen with a hole in the center held at constant tension.
Another device for measuring the tendency of plastic articles to fail under stress is disclosed by Haslett et al. in the SPE Journal (March 1964). The weight which produces the load on the specimen is located above the horizontal plane of the specimen. The disclosed device requires a ball bearing bushing to reduce the air friction of the loading shaft and an air cylinder to absorb the shock of weight's fall after fracture, while at the same time actuating a normally closed pressure switch to the timer to record the exact time of failure.
An apparatus for measuring flexural creep rupture is disclosed by Schmitz et al. in SPE, 17th ANTEC, Volume VII (January 1961). A beam is loaded with a weight from above in the center of a four-inch span. The apparatus records failure times automatically by means of a micro switch which is tripped after the bearing rod has moved 2.5 inches.
U.S. Pat. No. 3,710,616 to Smith et al. discloses an apparatus and method wherein a weighted rod stresses a plastic article in contact with a stress cracking agent and an electrically conductive liquid until the liquid penetrates the stress cracks which appear. Another method for measuring the environmental stress cracking of plastic materials is disclosed by Sieda et al. in U.S. Pat. No. 4,107,979 wherein the test piece is initially deformed in a test environment and an additional stress of a magnitude smaller than that of the stress already produced in the test piece is applied until the test piece fractures. The apparatus requires a means for placing the test piece under stress before the test begins.
A creep testing apparatus is disclosed in U.S. Pat. No. 2,506,048 to Van den Akker. The disclosed apparatus includes means for subjecting a test specimen to a tensional or flexural load over a prolonged period of time. At various intervals during the loading period, the deflection of the material is measured. The load is applied by a weight pan attached to a loading yoke which causes a weight distributing bar to apply the load to the specimen. The weight pan is located below the horizontal plane of the specimen.
The devices of the prior art are relatively large so that multiple testing units can occupy an inconvenient amount of space in a plant situation. Further, required components such as a significant amount of conductive liquid, means for initially deforming a test specimen and means for accurately measuring the deflection of a specimen make the prior art methods and devices inconvenient for quick, successive tests on a routine basis. When the weight producing the load is located above the horizontal plane of the test specimen, additional means are needed to maintain the weight in a fixed path and minimize friction so that successive test conditions can be duplicated for meaningful results.
It is an object of this invention to provide a sensitive method of measuring the environmental stress crack resistance (ESCR) of plastic articles which has a high level of reproducibility and a short turnaround time.
Another object of this invention is to provide a device for measuring the ESCR of plastic articles which is convenient to use and produces accurate, reproducible results.